Best overall, especially for checking SNPs across multiple samples: IGV. The major problem to me is IGV does not display inserted sequences (still true?).
Most beautiful: Tablet. It supports more formats, but does not provide enough information for eye-checking variant calls. I always find IGV to be more convenient for my tasks.
For ChIP-seq: SeqMonk. Heard good things about it; never used.
Occasionally handy: samtools' tview (when over slow network or for quick check). One of the few viewers that display inserted sequences (what else besides gap5?). But most users would prefer IGV instead.
The only assembly editor: gap5. You need to convert BAM to the gap5 format first.
Others: BamView (limited functionality), MagicViewer (not always working) and Gambit (not so mature as others). There are a few more but I forgot the names.